This study will investigate the impacts of timely and progressive Clean Heat regulations that have a short implementation cycle ending to be fully executed by 2015. The proposed project will examine the variance in residential exposure to black carbon (BC) before and after implementation and analyze the policy process. BC is produced by any incomplete combustion process and is a surrogate tracer of different combustion sources. In NYC, major sources of BC include space heating and high density vehicle traffic (especially trucks).1,9-19 A significant contributor to the city's current BC emissions are buildings that burn residual oil (No. 4 and No. 6) for heating.14,18-19 Previous research has demonstrated that residual oil represents a significant environmental and public health threat. These environmental hazards are linked to a variety of health problems including cardiovascular disease, respiratory illness and lung cancer. A recent policy measure issued by the New York City Department of Environmental Protection mandates conversion from No. 6 fuel to cleaner burning fuel sources including lower sulfur No. 2 fuel, biodiesel or natural gas. This public health law is intended to address widespread air pollution by reducing fine particulate matter (PM2.5) emissions that produce soot and black carbon in NYC. As NYC is the most populated city in the United States, this new regulation marks one of the largest and most comprehensive pieces of environmental, energy and public health policy in the nation in the past decade. This study will substantially improve the evidence base for the efficacy of environmental and public health policies. We will measure residential indoor/outdoor air quality prior to heating fuel conversion in the 2013-2014 heating season and one-year post-conversion in the 2014-2015 heating season (Aim 1). We will also use the NYC Clean Heat policy intervention as a case study and identify the contextual and process-level factors involved in the effective passage and implementation of these laws (Aim 2). Together the mixed method approaches in Aims 1 and 2 will yield contextualized results to demonstrate changes in domestic and neighborhood levels of fine particulate matter (PM2.5) associated with BC generated in buildings that burn residual oil (i.e., No. 6), but by legal mandate are obligated to convert to cleaner burning fuel sources before the 2014/2015 heating season. The proposed study will document changes associated with the Clean Heat regulations in a neighborhood context beset by asthma and other adverse health effects linked to BC exposure to address issues related to health disparities. If successful our innovative study design will advance our scientific knowledge about residential exposure changes to BC in the aftermath of this policy intervention and the impact on vulnerable populations. Responsive to NIEHS' Strategic Themes, the proposed study focuses on exposure research and health, the translation of science to public policy, health disparities, interdisciplinary approaches to environmental health, diversity and the broad dissemination of research findings.